1. Field of the Invention
The invention is directed to a wind powered generating device comprising a tube cluster, a collector assembly, and a turbine assembly which improves the efficiency of such devices.
2. Description of the Related Art
Wind-powered generators have been around for some time. In conventional wind-powered generators, a sustained ambient wind speed of 11-13 mph is required to attain xe2x80x9ccut-inxe2x80x9d speed (the point at which the turbine is generating sufficient power to be safely and efficiently placed on the grid). At cut-in speed, conventional turbines are generating only about 20% of their rated power, and they do not reach their peak rated power output until wind speeds reach 25-30 mph. This means that there are relatively few places in the world in which wind generators can be considered a reliable source of electricity.
Over the years, sophisticated control systems and blade designs have been developed to assure relatively stable output characteristics over a wide range of wind conditions, but despite a steady flow of incremental improvements, the need for an ambient wind speed of at least 11-13 mph persists. Before a site is considered to be commercially viable, it must reliably be subject to wind speeds much higher than those necessary for cut-in speed, consistently bringing the turbine up to or at least close to its full rated power. In the United States, there are limited areas where such conditions exist.
The problem of finding suitably windy sites is not presently the only issue that is hindering the growth of the wind power industry. With the height of the latest wind generators approaching 230 ft., wind farms utilizing present designs are increasingly becoming a hazard to migratory birds and private air traffic. Construction and maintenance costs are skyrocketing as these new machines tower to ever increasing heights, and discussions about noise and visual effects on the landscape are also becoming contentious.
A widely accepted, practical formula for estimating the power output of a wind turbine is as follows:
P=0.5xc3x97rhoxc3x97Axc3x97CPxc3x97V3
where
P=power in watts (746 watts=1 hp)(1,000 watts=1 kilowatt)
rho=air density (about 1.225 kg/m3 at sea level, less at higher altitudes)
A=the swept area of the rotor exposed to the wind (m2)
CP=Coefficient of performance (0.59 {the Betz Limit} is the maximum theoretically possible; 0.35 is considered to be a good design)
V=wind speed in meters/sec (20 mph=9 m/s)
Other related variables include:
Ng=generator efficiency (50% for a car alternator, 80% or possibly more for a permanent magnet generator or grid-connected induction generator)
Nb=gearbox/bearing efficiency (good designs can yield an efficiency as high as 95%)
From the above formula, it can be seen that the easiest way to increase the power output of a wind turbine is to increase the velocity of the air passing the capture area (the area swept by the turbine blades). Because power increases by the cube of V, even small increases in wind velocity within the capture area yield relatively large increases in power output. Unfortunately, manipulating the wind speed using conventional free-air designs is not possible, since, by definition, the wind speed is the ambient wind speed. If, however, the air speed passing the turbine blades could be accelerated, the following benefits would result:
1. Wind generators would reach both cut-in speed and full rated power at lower ambient wind speeds. This could result in raising large parts of the world by as much as a whole power class (as defined by the United States Department of Energy), meaning that many areas which are now considered unsuitable as wind sites would become available as viable sites. The resultant decentralization of generators would insure that the grid as a whole was less vulnerable to the uncertainties of local weather conditions.
2. Intermittency (the time that the turbine spends below its cut-in speed) would be reduced, and conversely, availability would increase, resulting in an increase in annual energy output. This increase in efficiency would lower the average cost of power generation, making wind even more competitive with other sources of electricity.
Furthermore, conventional free-air turbines are engineered to have a service life of between 20 and 24 years, with scheduled periodic maintenance and one major overhaul at some point in time near mid-life. One of the most persistent problems that has plagued the industry has been a rate of component failure, especially blade failure, which is higher, sometimes much higher, than that predicted by computer models. This disparity between predicted and actual component life has been suggested by engineers to be due in great measure to the sheer number of unpredictable variables in a free-air system. The speed of the wind typically increases as one rises above the frictional elements close to the ground. This means that the forces that are exerted on the blade components traveling through the top of the rotor arc are significantly greater than those at the bottom of the arc. In addition to the cyclic flexing of the blades as they are subjected to these differences in wind speeds, they are also subject to alternating states of compression and tension as they travel around the hub. Wind gusts, off-axis buffeting, and structural harmonics provide additional sources of chaotic loading to the system, stressing not just the blade set, but the rotor hub, gearbox, and all associated bearings.
The cost of refitting a 1 megawatt free-air turbine with a new blade set, which typically has a diameter of approximately 60 meters, can easily exceed $300,000 U.S. (1999), which is about one third of the installed cost of the unit. From this we can see that any improvements which are capable of extending the service life of the system have the potential to make wind energy a more competitive alternative to other forms of power generation.
Present tower designs also produce the undesirable effect of stroboscopic flicker, which occurs to a stationary viewer on the ground when each blade passes between the viewer and the sun. This effect can be annoying to residents living within view of the towers, especially at those times of day when the sun is low in the sky.
Early designs in power generating devices have taken various approaches to maximizing efficiency while considering related design parameters. U.S. Pat. No. 1,600,105 issued to Fonkiewicz in 1923 shows a power generating device with a vertical stack having a turbine within, and radially extending tunnels that communicate with the stack, the tunnels being located below the ground surface and having openings in the ground. U.S. Pat. No. 4,036,916 issued to Agsten in 1977 shows a wind driven electric power generator with an updraft natural draft cooling tower having a hyperbolic veil with a wind driven electric generator system positioned at a narrowed area of the hyperbolic veil. U.S. Pat. No. 581,311 issued to Scovel in 1897 shows a rotatable hood positioned on top of a tube containing fans, which rotates to capture wind and direct wind to the fans. U.S. Pat. No. 4,049,362 issued to Rineer in 1977 shows airfoil panels utilizing fabric to capture wind to generate power. Finally, U.S. Pat. No. 4,779,006 issued to Wortham in 1988 shows a hybrid solar-wind energy conversion system having a xe2x80x9cJxe2x80x9d shaped tubular stack with a generator fan positioned in a tube below the surface of the ground.
In general, however, none of these related art references utilize strong lightweight structures that are self-regulating and easily turn to face the incoming wind, redirecting a substantial portion of the kinetic energy present in the ambient air stream into a tube set, where the air is channeled into a below-ground turbine located at a narrowing in an output tube which takes advantage of the Venturi effect, enabling significant efficiency and operating capability even at low wind speeds.
An object of the invention is to create a device that will collect, redirect, and accelerate ambient air, then channel it to the capture area of a turbine, thereby surpassing the performance of a conventional wind turbine operating in free air, and other conventional designs, with minimal noise and environmental impact, allowing economical operation in areas that were infeasible with previous designs.
This object is achieved with a wind-powered generation device comprising a tube cluster, collector assemblies, and a turbine assembly where the tube cluster and turbine assembly are primarily underground, and the central outlet tube is narrowed/pinched at the center to increase the rate of airflow past the turbine by taking advantage of the known Venturi effect. Lightweight, self-regulating collector assemblies gather a much greater volume of air than could be captured by a turbine rotor assembly in free air while greatly reducing the variability in the speed of the wind passing the blades. The tube set which channels the collected air and accelerates it as it passes the rotor, combined with the rotor which operates on a plane parallel to the ground, creates a system which significantly reduces the amount of buffeting, tension-compression variability, asymmetrical loading, and other elements of component stress, both cyclic and non-periodic, that are major sources of fatigue-related structural failure. The resultant increase in reliability and service life, and the reduction in maintenance costs, effectively lower the per-kilowatt cost of generating energy. Additionally, the present design eliminates the flicker effect produced by existing tower designs because its turbine blade is underground.
In areas where wind energy may be marginal or intermittent but heat energy is abundant and readily available, an additional mechanism may be used to boost the efficiency of the system. The rising of warm air is a well-known phenomenon and hence heat injected into the air stream at the proper place in the main outlet tube would serve to boost the performance of the system. Two potential sources of heat are solar and geothermal.
Well-planned combinations of functions provide investors with an extra measure for profit, thus encouraging more investment in environmentally sound generating sources such as wind. For example, during periods of high wind and low demand, generators placed next to coastlines could be taken off of the power grid and put to other tasks, such as the purification and desalination of seawater, the creation of oxygen gas or hydrogen fuel for fuel cells and other hydrogen-powered equipment, and other valuable commodities that can be produced by way of electrolytic reactions.
The system may be tuned by varying parameters on the open tubes to promote phase cancellation of low-frequency acoustic energy (ranging from below 8 Hz to above 20 Hz). This may be needed for the following reason: because of the low rotational speeds of the turbine blades, the peak acoustic energy radiated by the current generation of turbines is in the infrasonic range (8-12 Hz) for large diameter turbines, and in the low-frequency end of the audible spectrum (20 Hz) for smaller turbines or those with multiple blades. Although powerful infrasonic waves were found by the U.S. military to have deleterious effects such as nausea, vomiting, and dizziness on humans, acoustic pulses at these frequencies are generally considered to be more of an annoyance than anything else and the problems they create are generally overcome by the use of ear plugs. Other mechanisms for dealing with this issue may be considered as well.